This invention relates to a knee joint, and, more particularly, to a patellar-femoral joint.
There are two joints in the knee which permit movement between the femur and the tibia. A weight-bearing joint occurs between the condyles of the femur and the superior end of the tibia, and a joint occurs between the patella or knee cap and the femur. The patellar ligament extends between the patella and the tibia and fixes the distance between the patella and the tibia. Accordingly, as the leg is flexed and extended, the patella slides up and down in the intercondylar groove of the femur. This is a so called gliding joint, and although not a true weight bearing joint is also subject to various forms of arthritis. As the articular surface of the patella thins and roughens either as a result of wear and tear or due to injury, friction increases, function decreases and pain occurs.
Not too many years ago the function of the patella was poorly understood and many believed that it was a vestige from some predecessor in the evolutionary tree and really had no function in man. This philosophy popularized the treatment of patellar-femoral arthritis by the removal of the patella. This is still the most commonly used treatment because there has been nothing else that provides better results.
An analysis of those knees that had been subjected to the removal of the patella revealed the major disability was a weakness in the last few degrees of extension. It is now appreciated that the patella functions both as a pulley to change the force vectors of the pull of the anterior thigh muscles on the knee and as a fulcrum for the lever action required. An additional consideration was that after the removal of the patella, the remaining tendon continued to move through the intercondylar notch and continued to worsen the arthritis on that surface.
In the normal function of the knee the patella rests in a concave groove between the condyles of the femur which is called the patellar bed of the distal femur. This bed and the opposing under surface of the patella are covered with articular cartilage and subject to arthritis. The opposing surfaces of this joint are not in contact with the weight bearing portion of the knee joint.
The intercondylar groove aids in stabilizing the patella and prevents medial and more significant lateral displacement of the patella. The tendency to laterally displace is inherent to the normal anatomy of the patellar ligament attachment and the formation of an angle between the quadriceps muscle pull and the site of the attachment, called the Q Angle.
With motion of the knee the patella moves through the intercondylar groove in articulation with the femoral bed and in so doing a different portion of the patella is in contact with a different portion of the bed during various positions. In the extended position the inferior portion of the patella is in contact with the superior part of the bed. In flexion the superior part of the patella is in contact with the inferior part of the patellar bed.
I have devised the construction of two components to be utilized for the total replacement of the patellar-femoral joint. This joint although not a weight bearing joint is subject to considerable compression forces between component bones. This force is exerted because of the pulley like function of the patella in changing the force vector of the muscle pull of the thigh muscles. Because of the angular relationship between the direction of the pull of the quadriceps muscle on the patella and that of the patella ligament (the Q Angle) and the fact that knee joint flexion and extension is not a pure hinge movement but has associated rotation means that a simple groove could not retain the patellar component without considerable restrictive force with adds friction and wear. Because the shortest distance between two points is a straight line, as pull is applied on the patella during extension, there would be a lateral shift of the patellar which would tend to displace it from the groove.
Rather than a groove, I propose a femoral component which would replace that portion of the surface of the femur which comes into contact with the patellar component. The femoral component has a concave surface which engages the patellar component and which is substantially wider than the patellar component so that the patellar component can move laterally and medially over the concave surface.